Inter‐relationships between Oxygen Evolution and Iridium Dissolution Mechanisms

Abstract The widespread utilization of proton exchange membrane (PEM) electrolyzers currently remains uncertain, as they rely on the use of highly scarce iridium as the only viable catalyst for the oxygen evolution reaction (OER), which is known to present the major energy losses of the process. Understanding the mechanistic origin of the different activities and stabilities of Ir‐based catalysts is, therefore, crucial for a scale‐up of green hydrogen production. It is known that structure influences the dissolution, which is the main degradation mechanism and shares common intermediates with the OER. In this Minireview, the state‐of‐the‐art understanding of dissolution and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of the OER. A perspective on future directions of investigation is also given.


Introduction
Increasing interest in renewable energy sources,asaresult of policies aiming to limit anthropogenic changes to the climate,demands the development of innovative technologies that would mitigate the intermittence of solar and wind energy.Storing excess energy in the form of chemical bonds, specifically as hydrogen, is currently considered one of the best options,which is confirmed by the increasing number of electrolyzer installations worldwide. [1] However,e fficient scale-up of this technology is currently still hindered by the low efficiency,w hich arises predominantly from the sluggish kinetics of the anodic oxygen evolution reaction (OER) and the high cost of iridium-the only metal that is able to resist the harsh, acidic conditions in the proton-exchange membrane (PEM) electrolyzers while showing ar elatively high activity. [2,3] Af undamental understanding of the mechanisms driving the reaction and degradation is of great importance for improving the performance of catalysts.Systematic studies of the OER mechanism on different noble metals can be traced back to the 1950s,w hen the first kinetic studies were published. [4][5][6][7][8] They revealed various possible reaction pathways,which lead to different performances of the Ir catalysts. Thefocus of investigations on the properties of the Ir catalysts has only recently started to slowly shift from activity towards stability.I nt his regard, the dissolution of active material is acknowledged as the predominant degradation mechanism. Theu nderstanding of the dissolution kinetics has advanced with the development of experimental techniques,s uch as inductively coupled plasma mass spectrometry (ICP-MS), which combine electrochemical measurements with the online detection of dissolved species. [9,10] Thed issolution kinetics are affected by both the operational parameters and physicochemical properties of the catalyst. Specifically,inthe case of iridium, the less active rutile (IrO 2 )form is known to be stable under the OER conditions,w hereas its amorphous analogues exhibit higher activity for oxygen evolution, but also lower stability.From the dissolution studies,itisapparent that the OER and dissolution mechanisms are intertwined through ac ommon reaction intermediate. [11,12] Additionally, the participation in the OER of oxygen from the oxide lattice can lead to destabilization of the Ir-oxide structure and result in dissolution.
As aconsequence of the interlinking of the two reactions, we start this Minireview with ab rief discussion on the mechanism of the OER, which has been studied in further depth in other recent reviews. [13][14][15] This sets our starting point for am ore detailed analysis of the state-of-the-art understanding of the dissolution mechanisms of different Ir-based catalysts-the core focus of this Minireview-which is generally overlooked in other overarching activity-and stabilityoriented publications. [16,17] Thes ummary of the most recent literature is concluded with ap erspective on the future challenges and strategies to overcome them.

Mechanism of the Oxygen Evolution Reaction on Ir-Based Catalysts
TheOER is acomplex electrochemical reaction involving four electron and proton transfers and at least two reaction intermediates. [18] Thef irst experimental studies of the OER on different noble metals,w hich relied only on electrochemical methods such as Tafel analysis,d educed that the OER on iridium occurs by an electrochemical oxide pathway, [19,20] which was suggested by Bockris ( Figure 1a). [4] Since then, various studies have aimed to broaden the understanding of the mechanistic pathways,e ach using ad ifferent approach.
Kçtz et al. published one of the first ex situ X-ray photoemission spectroscopy (XPS) studies of IrO 2 films and, based on the constant ratio between oxygen and iridium The widespread utilization of proton exchange membrane (PEM) electrolyzersc urrently remains uncertain, as they rely on the use of highly scarce iridium as the only viable catalyst for the oxygen evolution reaction (OER), whichi sknown to present the major energy losses of the process.U nderstanding the mechanistic origin of the different activities and stabilities of Ir-based catalysts is,therefore, crucial for ascale-up of green hydrogen production. It is known that structure influences the dissolution, whichisthe main degradation mechanismand shares common intermediates with the OER. In this Minireview,the state-of-the-art understanding of dissolution and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of the OER. A perspective on future directions of investigation is also given. at all the studied potentials,proposed acationic catalytic cycle involving an electrochemical oxide pathway,where iridium is oxidized to an Ir VI intermediate,w hich is reduced in the following reaction step back to Ir IV with as imultaneous release of an oxygen molecule (Figure 1b). [21] More recent in situ XPS [22] and X-ray absorption spectroscopy (XAS) [23] studies on iridium oxide nanoparticles and hydrated iridium oxide films,respectively,have,incontrast, suggested aIr V -Ir III transition, with the presence of both oxidation states under the OER conditions. [23] This mechanism was further supported by Sivasankar et al.,who for the first time detected an Ir-OOH intermediate by probing iridium oxide nanoclusters using infrared (IR) spectroscopy (Figure 1c). [24] Cationic redox processes,o nw hich these studies were based, have traditionally been thought to be the main source of charge storage.
This view was challenged by the discovery of anion-driven capacity storage in Li-ion battery technology. [25,26] Theidea of an anionic redox mechanism was soon applied to the field of electrocatalysis.Itwas demonstrated that shifting the p-band of the oxygen atom closer to the Fermi level in metal oxides with ah ighly covalent network can trigger the redox activity of the lattice oxygen atoms (Figure 1d). [27] Theanionic redox mechanism was experimentally uncovered in different studies. [28][29][30][31] Saveleva et al. aimed to reveal auniversal mechanism by using near ambient pressure XPS (NAP-XPS) and XAS, combined with ab initio calculations,tostudy two different Irbased catalysts:thermally oxidized IrO 2 and electrochemical amorphous iridium oxide nanoparticles. [30] Thea uthors detected the formation of oxyl O À species on both rutile and the structurally more flexible IrO x nanoparticles,a nd concluded that the involvement of anionic lattice oxygen atoms in the OER is universal, regardless of the structure of the catalyst.
Thei nvestigation of oxidation-state changes of the catalyst is important, as they are directly related to the charge-storage mechanism, which was recently shown to be the driving force of the reaction. [32] Nevertheless,t he latest studies,w hich employ combinations of in situ spectroscopic techniques and theoretical calculations,are now showing that focusing entirely on either ac ationic or anionic mechanism probably does not describe the full picture.Ithas been shown that, as ar esult of as trong hybridization of the iridium and oxygen orbitals,the positive charge is shared between cations and anions. [33] Specifically,t he formation of the reactive oxyl species depends on the oxidation state of the iridium. [34] As the catalyst is exposed to ah igh anodic potential, the accumulation of positive charge in electron-deficient oxygen species results in ad ecrease of the activation energy for the nucleophilic attack of water molecules and the formation of an OÀOb ond, which is currently understood to be the ratedetermining step of the OER (Figure 1e). [18,32,34,35] 3. Mechanistic Understanding of Ir Dissolution in the OER One of the first dissolution studies comparing different noble metals,which used the newly developed scanning flow cell (SFC) coupled with inductively coupled plasma mass spectrometry (ICP-MS), [9,36] showed that dissolution of all the studied metals increases as the rate of the OER accelerates, which already suggests ad irect interconnection between the activity and stability of OER catalysts. [2] Thee xtent of dissolution was,h owever,v ery different depending on the nature of the investigated metal. Iridium was found to have an intermediate response,both in terms of activity and stability. Thea uthors showed that the stability can be estimated from the Tafel slopes,w hich indicate different operating OER mechanisms.O nm etals with ah igh Tafel slope,s uch as platinum, the OER is expected to proceed through an adsorbate route,w hich does not involve the participation of the oxide lattice in the reaction and does not disturb the surface of the catalysts,w hile on metals with as maller Tafel slope,t he OER involves the participation of at hick oxide layer, which results in its destabilization and higher dissolution of the catalyst ( Figure 2a). Interestingly,the authors did not find ac orrelation between the onset of the OER and dissolution. This implies that the activity and stability of metals are not necessarily in ar everse relationship and that ah ighly active and durable catalyst could potentially exist. Such af inding is,h owever,n ot in agreement with the conclusions from as imilar study published the same year that compared the activity and stability of five different metal oxides ( Figure 2b). [37] In this case,t he authors observed trends in the OER overpotential and the dissolution of metal cations and suggested that as trong link exists between the two parameters.F urthermore,b yc omparing highly defective polycrystalline Ru and Ir electrodes to single-crystalline model electrodes with aw ell-defined surface,t hey deduced that the nature of the oxide and surface defects control the activity and stability of the catalyst. Based on the results,t hey concluded that since activity and stability are inversely related, the ideal OER catalyst should thus display abalance between them by dissolving "neither too fast nor too slow".
Thed issolution of different iridium oxides was further tested to determine which parameters result in the improved stability of different oxides.I tw as shown that thermal treatment of either chemically [38] or electrochemically [39] prepared iridium oxides resulted in improved stability and decreased activity.T his trend was explained by changed stoichiometries and increased crystallinity after annealing. Thel ower dissolution resistance of oxides heat-treated between 100 and 300 8 8Cs howed that, besides the crystal Figure 2. a) Correlation between the stability of the catalyst with aT afel slope and the operatingO ER mechanism. [2] Copyright2 014, John Wiley and Sons. b) Activity-stability trend for different metal oxides, showing an inverse relationship between the reactivity in the OER and the durability of the catalyst. [37] Copyright 2014, American Chemical Society. Figure 1. a) Electrochemical oxide pathway. [13] Copyright 2016, John Wiley and Sons. b) Cationic redox mechanism proposed by Kçtz et al. [21] Copyright 1984, IOP Publishing. c) Scheme of the OER, including the formation of an OOH intermediate, as detected by Sivasankar et al. [24] Copyright 2011, AmericanC hemical Society.d )Schematic representation of O2pbands penetrating into Ir dorbitals and triggering an anionic redox process. [27] Copyright 2016, Springer Nature. e) OER scheme showing the formation of oxyl species, as aresult of hybridization of Ir and O orbitals, which are prone to nucleophilic attack by water and the formation of an OÀObond. [33] Copyright2 019, Elsevier. structure,t he hydration and conductivity of the oxide play as ignificant role in the activity-stability properties (Figure 3a). Thee ffect of different crystal structures was further shown in as tudy investigating the activity and stability of iridium and ruthenium as well as their oxides. [40][41][42] As discussed above,t he participation of oxide in the OER promotes the dissolution of the catalyst. In ar ecent publication, Hao et al. showed that the active involvement of lattice oxygen can be manipulated by tuning the electronic structure of the catalyst. Thei nvolvement of lattice oxygen in O À O bond formation can be suppressed by changing the formation energy of oxygen vacancies (V O ;F igure 3b). [40] Practically, this hypothesis was demonstrated by doping the RuO 2 lattice with Wand Er.T his approach was shown to be exceptionally effective,a st he downshift in the oxygen 2p band resulted in the representative catalyst displaying long-term dissolution stability over at least 500 h. After analyzing the aforementioned study,E xner pointed out that calculating V O could potentially be used in future theoretical studies as as tability metric. [43] Theneed for universal stability descriptors is particularly relevant for the experimental evaluation of novel materials.It is well-documented that parameters such as the accumulation of oxygen bubbles [44] or passivation of the backing electrode [45] can lead to erroneous interpretations of traditional stability measurements with techniques such as rotating disc electrodes (RDEs). Additionally,t he choice of experimental parameters,t hat is,d ynamic or stationary measurement procedures, [46] loading effect, and the unknown active surface area of the catalyst further complicate the dissolution and thus stability evaluation of the investigated material. To overcome such limitations,n ovel metrics such as the Snumber [47] and equivalent activity-stability factor (ASF) [42] are emerging.Both are defined as the ratio between the number of evolved oxygen molecules and dissolved iridium atoms. These metrics explicitly show the correlation between the activity and stability of different catalysts,which can be used to compare the different stabilities of newly designed materials. [48][49][50][51][52][53] However,i ti si mportant to emphasize that unifying the experimental parameters is crucial, as different protocols,that is,cycling or potential, affect the dissolution by triggering transient or steady-state dissolution, and thus affect the S-numbers. [2,54] To further elucidate the dissolution of iridium over abroader potential range,aseries of systematic examinations of the dissolution of both ab are metallic iridium disk and electrochemically grown iridium oxide were carried out by Cherevko et al. [11,55] It was shown that the dissolution of hydrous iridium oxide depends both on the potential of the electrode and the thickness of the oxide layer;anincrease in both parameters leads to enhanced dissolution. By combining the experimental data with the OER mechanisms already proposed in the literature, [21,23] the authors suggested that Ir might dissolve via either Ir III or Ir VI intermediates,depending on the structure of the catalyst.
To confirm this hypothesis,K asian et al. combined dissolution measurements obtained using SFC-ICP-MS with the detection of volatile intermediates and products of the OER by online electrochemical mass spectrometry (OLEMS). By investigating three different iridium anodes, namely metallic iridium as well as reactively sputtered and thermal iridium oxide,the authors detected Ir VI intermediates for the first time. [12] They simultaneously measured the dissolved iridium and formation of O 2 and IrO 3 at 5, 10, 15, and 20 mA cm À2 .O nt hermal oxide,w hich displays al ower reactivity towards the OER, the formation of the volatile IrO 3 intermediate was already detected at the lowest current density,w hereas on the other two more-active electrodes it was possible to detect it only at the highest current densities after the potential on the anode exceeded 1.6 V. Thestate-ofthe-art understanding of the OER and dissolution were combined into ap otential-dependent universal scheme, centered on ac ationic redox mechanism (Figure 4). Regardless of the material, the first step of the OER, marked with blue arrows,isthe elimination of water and adsorption of an OH radical on the surface of the catalyst, which is accompanied by the oxidation of the iridium center and leads to the formation of the Ir V O 2 (OH) intermediate.T he next steps depend on the electrode potential, which is determined by the Figure 3. a) Effect on the activity and stability of thermally treating hydrous iridium oxide films. [39] b) Stabilization of the RuO 2 lattice by increasing the formation energy of oxygen vacancies. [40] Figure 4. Universal mechanism correlating both the OER and dissolution pathways proposed by Kasian et al. [12] Copyright2 018, John Wiley and Sons. nature of the electrode.I ft he OER is catalyzed by thermal oxide,t he required potential is high enough for further oxidation of iridium to Ir VI O 3 .T his intermediate can then either decompose to O 2 and IrO 2 to close the catalytic cycle or react with water and dissolve as IrO 4 2À .C onsidering the relatively low dissolution of the thermal oxide,i tw as suggested that hydrolysis is kinetically suppressed, which could explain the superior stability of crystalline iridium oxide.I nt he case of more-active materials,t he applied potential is not high enough to further oxidize the iridium. Instead, the OER cycle is closed by decomposition of the Ir V O 2 (OH) intermediate with the evolution of an O 2 molecule and formation of the HIr III O 2 species,w hich can either dissolve as Ir 3+ or be further oxidized to IrO 2 .Aprevious study by Cherevko et al. [11] already suggested the formation of this Ir III intermediate to be the origin of the lower stability of metallic and hydrous iridium oxide catalysts.W hent he current densities are high enough to exceed the potential required for the oxidation of iridium further to Ir VI ,t he pathway marked with red arrows also becomes relevant for the more-active catalysts.H ere,d issolution through the formation of IrO 4 2À might, however, not be equally hindered kinetically,s ince Geiger et al. showed that, at potentials above 1.8 V, metallic iridium was already completely dissolved after 10 minutes. [47] Assuming that the proposed mechanism is operative,two different dissolution pathways,a ccompanying the OER, are possible for different catalysts.T he first dissolution route involves the unstable Ir III intermediate,w hich is formed on more active catalysts,s uch as hydrous iridium oxide and metallic iridium. Thei nvolvement of this intermediate was experimentally proven before, [22,23] and is usually assumed to be the reason for the poor stability of these materials.B ased on the observed high level of dissolution, it was concluded that the dissolution of Ir III is kinetically faster than further oxidation to IrO 2 .T he second dissolution pathway involves the hydrolysis of IrO 3 .T he third route,w hich should be mentioned as it also leads to degradation of catalyst but is, however, not directly related to the OER, is the inevitable dissolution of metallic iridium that accompanies the formation of protective passive oxide on the surface of ametal when it is exposed to high anodic potentials. [56] Thee xtent of dissolution depends on the nature of the metal, specifically on the cohesive energy and adsorption energy of oxygen, [57] as well as the surface structure. [58] Therelevance of the proposed general mechanism was evaluated by ab initio molecular dynamics simulations. [59] Thea uthors confirmed the thermodynamic stability of the Ir V intermediate over ar elatively broad potential window,w ith the Ir V intermediate being further transformed into IrO 3 at high anodic potentials and HIrO 2 at lower potentials.W hen further simulating the detachment of the Ir III intermediate from the surface,t he authors found that iridium can either deposit back on the surface of the oxide or dissolve as Ir(OH) 3 ,with the kinetics of redeposition being faster than the dissolution. Amorphization of the surface was experimentally observed and is thus in line with the calculations. [60,61] When considering the second dissolution route through IrO 3 formation, it was shown that the reactivity of this intermediate towards the OER was higher than that of the IrO 2 (110) surface.
Interestingly,n om echanistic study published to date has correlated dissolution with the anionic redox process.V elasco-Velez et al. recently showed that the anionic mechanism, that is,active participation of O À in the OER, depends on the presence of electron-deficient Ir V sites in IrO x . [34] In the future,s uch studies should also be done on rutile nanoparticles,a st he formation of Ir VI ,a sd etected by Kasian et al., [12] still needs to be explained in the context of an anionic redox process.S aveleva et al. detected O À in rutile and concluded that this electrophilic species might be an intermediate of the OER, regardless of the structure of the catalyst. [30] Looking from both perspectives,i tm ight be suggested that, due to strong hybridization between the oxygen and iridium orbitals,t he anionic and cationic mechanisms cannot be discussed separately.N evertheless,i fi ti s assumed that the O À intermediate is stable,t he scheme proposed by Kasian et al. is still feasible and will only need to be complemented with O À containing intermediates of an OER catalyzed by iridium oxides.
Thep resence of unstable intermediates promotes the dissolution of catalysts during the OER. Based on the possible presented reaction pathways,i tc an be concluded that the OER and dissolution are two parallel reactions with acommon intermediate.With this in mind, it may be possible to suppress one without impacting the other. However, the dissolution of the Ir III or Ir VI intermediates is not fully understood and it is not yet known whether the reaction is chemical or electrochemical. Nevertheless,the formation and lifetime of intermediates depend on the potential. As the OER progresses,the concentration of protons in the pores of the oxide can significantly increase,w hich could lead to enhanced dissolution of the less-stable intermediates.Finding the conditions where dissolution would be suppressed is, therefore,crucial to forthcoming stability-related studies.This could be achieved, for example,with achange in the pH value of the electrolyte.A dditionally,t he stabilization of the IrO 3 intermediate has already been demonstrated experimentally, specifically through aproton intercalation mechanism. [62]

Lattice Oxygen Evolution Reaction and Its Implications on the Stability of Ir Oxides
Thep articipation of lattice oxygen was mentioned repeatedly in the previous sections.I th as been shown that the less-stable catalysts,such as Ru and Au,are covered by athick layer of surface oxide,w hich actively participates in the reaction. [63][64][65] However, it should be noted that this process was found to be structure-dependent. [66] Thei nvolvement of the lattice is known to trigger enhanced dissolution. [2] Thefirst study,w hich quantified the extent of the involvement of the oxide layer in the OER on Ir, was published by Fierro et al. [67] Thea uthors combined differential electrochemical mass spectrometry (DEMS) and isotope labeling to detect oxygen that evolved from the oxide layer in Ti/IrO 2 .T hrough the detection of species with m/z 32 and 34, it was confirmed that oxygen is indeed partially evolved from the lattice.However, the study did not correlate this phenomenon with the possible destabilization of the structure through the formation of vacancies after the release of oxygen atoms.R ecently,t wo studies extended this experimental approach with additional simultaneous dissolution measurements. [47,68] Geiger et al. examined the stability of different Ir-based catalysts,n amely highly active perovskites,a morphous IrO x ,m etallic iridium, and rutile IrO 2 .T hey found that the rate of dissolution depends on the structure of the catalyst. Non-noble elements present in the perovskites dissolved immediately after immersion in the acidic electrolyte,l eaving behind amorphous,h ighly hydrated iridium oxide resulting from the collapse of the originally present iridium octahedral framework. Theordered structure with predominately edge-sharing oxygen atoms transformed into an amorphous structure with an increased number of corner-sharing oxygen atoms,w hich resulted in the enhanced dissolution of iridium. Additional isotope-labeling experiments on rutile and hydrous iridium oxide thin films provided information on the involvement of the activated, corner-sharing oxygen atoms in the mechanism of the OER (Figure 5a). Thea uthors concluded that the involvement of lattice oxygen in the OER depends on the structure and that the overall stability of different oxides is determined firstly by the stability of intermediates,w hich could be higher for the rutile structure compared to the amorphous oxides,a nd secondly by the ratio between the edge-and corner-sharing iridium octahedra, which was also in line with the observations of Willinger et al. [69] Here,S TEM and EELS analysis gave insight into the structural origin of the high activity of amorphous oxides.T he authors observed the presence of interconnected hollandite-like motifs,i n which oxygen is evolved through as o-called "paddle-wheel" mechanism ( Figure 5b). Additionally,t hey assumed that the presence of K + ions in the active phase could stabilize the open hollandite structure.

Lattice Oxygen Evolution Reaction and Stability of Hydrous IrO x
As tudy by Kasian et al. further aimed to quantitatively assess the contribution of the oxygen evolved from the lattice to the overall OER. [68] Thea uthors combined SFC-ICP-MS and OLEMS measurements with the atomic-scale structural characterization technique atom probe tomography (APT), which was previously used to unveil the structure of electrochemically grown iridium oxide under galvanostatic conditions. [70] Hydrous Ir 18  Detection of these last two molecules in measurements on hydrous oxide directly confirm the participation of lattice oxygen atoms,w hich results in the destabilization of the oxide structure and its higher dissolution, compared to reactively sputtered iridium oxide.I nt he latter case,t he dissolution was an order of magnitude lower with anegligible concentration of oxygen evolved through the participation of lattice oxygen atoms.T his conclusion could, however, originate from technical limitations arising from the concentrations of the evolved species being below the detection limit. APT analysis of both oxides was used to correlate the difference in the structure and stability and revealed that hydrous oxide nanopores are covered with alayer consisting of Ir-O and OH in an approximate 1:1ratio. This finding suggests that hydrous iridium oxide consists of Ir III -OOH species,w hich can by themselves act as the OER precursor.I ndeed, it was found that the ratio between the dissolution and the evolution of oxygen through ar ecombination of two lattice oxygen atoms was constant, which directly confirms their previously suggested correlation. Additionally,O Hg roups present in the hydrous layer stabilize the Ir III species in the oxide which can serve as the precursor for the peroxide route,thereby leading to evolution of O 16 O 18 .O nt he basis of the APT experiments,i tc an be suggested that the Ir III -OOH species could actually be the degradation intermediate HIrO 2 (Figure 4) and that its degradation may be accompanied by the release of an oxygen molecule.I nr eactively sputtered oxide,h owever,t he presence of such species was not detected, which is in line with its higher stability.Q uantitatively,o nh ydrous iridium oxide, approximately 0.05 %o fa ll oxygen molecules are produced by the peroxide route,w hereas only 0.01 %o ft he oxygen molecules originate from the recombination of two oxygen atoms from the lattice.

Lattice Oxygen Evolution Reaction and Stability of Rutile IrO 2
As mentioned previously,t he technical limitations of techniques such as OLEMS can lead to inaccurate conclu- Figure 5. a) Ordered rutile structure with edge-sharing octahedraand hydrated amorphous structure with activated corner-sharing oxygen atoms. [47] Copyright 2018, Springer Nature. b) "Paddle wheel" reaction scheme, proposedb yWillinger et al. [69] Copyright 2017, American Chemical Society.
sions when investigating more-stable oxides.T he studies highlighted in the previous sections have in general concluded that the rutile lattice does not participate in the OER, which could explain its higher stability. [66,68] However,t oo vercome possible detection-related limitations and test whether the exchange of oxygen anions is nevertheless possible in the case of more rigid structures,adifferent approach was used in the study reported by Schweinar et al. [71] Instead of detecting the evolved oxygen, the authors used an isotope-labeled reactively sputtered iridium oxide thin film, anodically polarized it at 1mAcm À2 for 10 minutes in anon-labeled electrolyte,and afterwards estimated the proportion of exchanged oxygen atoms by APT.T he analysis revealed asignificant increase in the O 16 species in the top 2.5 nm of the film, which was direct confirmation of the active involvement of the lattice in the reaction. Theo verall electrochemically active volume of the catalyst was nonetheless significantly lower than in hydrous iridium oxide,which explains the higher stability of rutile.The active involvement of oxygen from the rutile lattice was recently corroborated in ad ynamic OER operation through observation of an increased Ir-Ir interaction in IrO 2 by inoperando XAS measurements. [72] After the release of oxygen from the lattice,t he rearrangement of the structure leads to ad ecreased distance between the Ir atoms,w hich was proposed to be the origin of the higher stability of rutile. Structural changes in the lattice also lead to stronger IrÀO bonds,w hich could explain the lower activity of IrO 2 .T his is in line with the DFT calculations by Man et al. [73] which showed that the origin of the overpotential in Ir is the very strong binding of O. Foramore thorough understanding of the effect of surface orientation on stability and its effect on the participation of the oxide in the reaction, studies on the single-crystalline models,m ostly carried out to date on ruthenium oxide surfaces, [74] should in the future be carried out on iridium oxides. [75,76]

Concluding Remarks and Perspective
After almost adecade of fundamental dissolution-oriented studies on model systems,such as metallic iridium disk or thin films,t he processes driving the degradation of OER catalysts are now generally well-understood. Regardless of its nature,t he conditions of the OER generally affect the stability of the oxide structure,a lthough not to the same extent. Figure 7s ummarizes different processes that trigger the dissolution. [71] Thep articipation of either one (a) or two (b) oxygen atoms in the OER result in the destabilization of the lattice.This occurs more frequently on amorphous oxides, as their structure is more flexible,w ith ac onsiderably larger catalytically active volume with intercalated water molecules and the occupancyo fa ctivated oxygen atoms and Ir III -OOH species,w hich can by themselves act as OER precursors.I n reactively sputtered oxides,t he kinetics of lattice oxygen exchange is slower,but nevertheless present. It was previously suggested that the rate of oxygen exchange could potentially serve as am etric for evaluating the stability of different oxides. [71] Thevacancies that are created after the removal of oxygen can be refilled through either the adsorption of awater molecule or the migration of bulk oxygen atoms.This  Figure 7. Processes on the surface of iridium oxide catalysts under OER conditions. [71] Copyright 2020, American Chemical Society.
inevitably results in surface reconstruction that can further enhance dissolution, for example,t hrough the oxidation of defects (c). Oxygen from the lattice can also be exchanged by oxygen from the water (d). This process is not expected to be particularly destructive;h owever,i ts till requires bond rupture and formation, which would lead to destabilization of the surface.T he formation of the unstable Ir III species under the OER conditions results in dissolution of this intermediate.A fterwards,t he dissolved species can be redeposited back to the surface (e) and thus boost the amorphization of the surface and increase the possibility for further dissolution. Despite various surface processes taking place during the OER, dissolution measurements have, however, shown that the OER occurs on iridium-based catalysts,r egardless of the structure,p redominantly through the decomposition of water (f);t his makes iridium-based catalysts currently the catalysts of choice for the OER in acidic media.
However,t he question still remains:c an the acquired knowledge now be transferred to non-model systems? [77,78] When comparing the S-numbers of anhydrous ruthenium oxide,m easured in either aqueous electrolyte by SFC-ICP-MS or extracted from the PEM stack, the calculated lifetimes differed by more than two orders of magnitude.This suggests that dissolution measurements generally overestimate the dissolution of catalysts in the OER (Figure 8a). [47] This could originate from adifferent acidity in the PEM stack compared to the half-cell investigations,where an electrolyte with pH 1 is generally used. Additionally,t he diffusivity of dissolved ions out of the membrane or their deposition in the membrane or cathode could result in al ower dissolution of the catalyst. First attempts to evaluate the effect of different parameters on the dissolution rate were presented in astudy recently published by Knçppel et al. [79] Thes tudy aimed to test different parameters that differ between the model aqueous system and membrane electrode assembly (MEA). Ther esults revealed that the main source of the higher dissolution in model systems is the overestimated acidity and the stabilization in real devices over time (Figure 8b).
These results confirm the effect of the pH value on the dissolution of iridium under OER conditions.A sw as discussed above,t he instability of Ir III intermediates in the OER and dissolution pathways on more active catalysts leads to an enhanced dissolution of these materials compared to rutile IrO 2 .The observed suppressed dissolution at higher pH values implies that the stability of this intermediate could be pH-dependent. Future studies should, therefore,focus on the effect of acidity on the stability and activity of iridium-based catalysts.A lthough the effect of acidity on the activity was previously shown for various OER catalysts, [80][81][82] the stability was overlooked. Furthermore,adeeper understanding of the effective pH value under working conditions in the PEM electrolyzer should be obtained.
Results showcasing the discrepancies between model and real systems pave the way for future studies on the topic, which should aim to close the gap between them. Although we believe that half-cell measurements in aqueous systems can be used to estimate the stability of not only Ir-based novel materials,t echniques resembling am ore realistic environment of MEAs should be concomitantly developed for the evaluation of parameters,s uch as loading effect, binder content, and impact of 3D architecture,t hat affect the performance of the catalyst. Moreover,testing under realistic current densities is also desired, but currently unavailable with traditional techniques such as RDE. Setups such as the gas diffusion electrode (GDE) could be used for such studies, [83,84] as recently employed for the evaluation of OER catalysts. [84] However, realistic current densities have not yet been achieved, predominantly because of the mass transport limitations that still need to be overcome. Figure 8. a) Comparison between S-numbers obtained in aqueous electrolytes by SCF-ICP-MSmeasurements and extracted from the PEM stack. [47] b) Effect of pH and stabilization over time on the calculated S-numberi nanaqueous model system and MEA. [79] c) Difference in bubble accumulationi nRDE and MEA during OER. [98] Angewandte Chemie It was shown that stability predominantly depends on the structure of the catalysts.T his is especially crucial when nanoparticles with more surface defects such as vacancies, steps,kinks,and grain boundaries are considered. Their effect on the dissolution should, therefore,b em ore thoroughly studied in future studies. [85] Ad issolution study by Jovanovič et al. [86] on different iridium-based nanoparticles showed some discrepancyf rom the disk measurements carried out by Cherevko et al., [11,55] which was attributed to ap ossible particle size effect. Thelong-term stability of even crystalline IrO 2 nanoparticles could potentially be questioned, as Schweinar et al. [71] showed that the top 2.5 nm is actively involved in the OER, which could be detrimental for nanoparticles.A st he nanoparticles are often anchored on the conductive oxide supports,t heir dissolution behavior can be additionally altered through strong metal-support interactions (SMSIs). [87] This effect is increasingly gaining attention as it can minimize Ir dissolution. [53,88] It is,h owever, of immense importance to also consider the corrosion resistance and conductivity of the support material, as it has been demonstrated that it can affect both the activity and stability of catalytic material. [89][90][91] Advanced electron microscopy techniques,s uch as in situ transmission electron microscopy (TEM) [92] and identical location TEM (IL-TEM), [93] should be developed to observe compositional, structural, and morphological changes in the nanoparticles during the OER. Whereas the unambiguous interpretation of the obtained data is limited predominantly by the interaction of the electron beam with the electrolyte in liquid in situ TEM, IL-TEM is now ag enerally well-established technique for the atomic-scale observation of nanoparticulate electrocatalysts. [94,95] Only recently an ovel method, namely am odified floating electrode (MFE), was developed and applied to the oxygen reduction reaction (ORR). This technique enables facile handling of the delicate TEM grids and operation under realistic current densities. [96] Thechallenge which still needs to be overcome to efficiently use MFE, GDE, or TF-RDE for studying the OER is efficient removal of the generated oxygen bubbles.Their effect was shown in publications by El-Sayed and co-workers. [44,97,98] In their most recent contribution, the authors attributed the more efficient removal of bubbles in MEA to the generation of an O 2 pressure gradient in the membrane/electrode/porous transport layer (PTL) interface and electro-osmotic drag of water to the membrane, which cannot be stimulated in an RDE setup because of different configurations (Figure 8c). Thus,t od esign an aqueous system where realistic current densities could be reached, these dynamic processes in the catalyst layer should be stimulated.